The field of the invention is internal combustion engines fueled by biogas, and particularly, applications such as electrical generator sets in which the engine is operated intermittently.
Biogas is produced by the anaerobic digestion of hydrocarbons. Organic waste materials such as wood byproducts (wood chips, sawdust, bark, etc.), public and private landfills, food processing byproducts, waste treatment plants and farm waste, produce biogas comprised mainly of methane and carbon dioxide. Any organic waste that can be processed through an anaerobic digester will produce methane in concentrations ranging from 50% to 80%. Unfortunately, there are other constituents produced by the anaerobic digestion of hydrocarbons, some that are inert and others that can form acids with the water vapor being produced. For example, small quantities of hydrogen sulfide may be produced and form sulfuric acid, and mercaptans, which are alcohols with an OH radical, can also form sulfuric acid. Carbon dioxide, which is produced in significant amounts is an ingredient for carbonic acid.
Biogas is used commercially in three ways. First, it can be burned directly to produce heat. This is the simplest use, since the various constituents are of little concern in the combustion process. Biogas may also be filtered and pumped into natural gas pipelines for widespread distribution. And finally the biogas may be filtered and applied to fuel an internal combustion engine. This latter application is particularly useful because the engine may power a generator which produces electrical energy.
When used to fuel an internal combustion engine, the constituents in biogas can significantly shorten the useful life of the engine. This is particularly true when the engine is operated intermittently. During shutdown, the last few strokes of the engine piston intakes unburned biogas. The various acids discussed above soon form and corrode the surfaces of the cylinder walls, the piston rings, valve seats and spark plug. These acids leak by the corroded rings into the oil sump where they are pumped throughout the engine. The contaminated oil is particularly corrosive for copper and lead alloy components. As this corrosive process progresses, the engine becomes more difficult to start until it eventually fails completely.
A typical prior art biogas driven system is shown schematically in FIG. 1, where the biogas is produced in a digester 10 and stored in a vessel 12. The pressure of the biogas can vary considerably, and a regulator 14 is employed to maintain the gas pressure at a relatively constant level at the input 16 of a carburetor 18. The biogas is mixed with combustion air in the carburetor 18 and injected into the engine cylinder 20. This basic system may also include a pump which boosts the biogas pressure in applications where the pressure is less than 4 inches of water, and it may include gas filters when corrosive constituents are a known problem. For example, hydrogen sulfide can be removed using a variety of liquid absorbents and/or solid phase oxidants, however, these chemical solutions are expensive and capital intensive. Several dry processes are also available using particles of either activated carbon, molecular sieve, or iron sponge to remove the sulfides from biogas. These are also very expensive.